One method of disposing of municipal solid waste and industrial solid waste is to incinerate it. Incinerators have been developed in Europe whereby the bulky solid waste is burned on grates of a wide variety of designs. These incinerators are large, expensive and inefficient, requiring large amounts of excess air to effectively burn the solid waste to completion.
Municipal solid waste is composed of large quantities of paper and plastic, as well as inerts such as bottles, cans, and various foreign objects which are not burned, but whose state may be physically or chemically altered. On average the waste composition is as follows: combustibles 44%, moisture 32%, and inerts 24%. Its average net heating value is perhaps 3500 BTU/lb. It can be expected that the composition of municipal solid waste has great variability from load to load; for example, one load may contain predominantly wet leaves while another load may be laced with magnesium chips, and yet another may contain discarded home appliances such as a refrigerator or a washing machine. Each of the above loads would have dramatically different combustion characteristics. In addition, the combusted materials may create air pollutants in the exhaust gas, such as SO.sub.2 from the sulfur in rubber wastes and HCl from the chlorine in PVC plastics.
Industrial solid waste can contain all of the same constituents as are found in municipal solid waste, but usually its composition is more closely defined, because it is generated from a given manufacturing process of a single business activity. An incinerator that can successfully burn heterogeneous municipal solid waste can usually burn almost any tupical homogeneous industrial solid waste. Hereafter I will refer to municipal solid waste. This term is meant to include primarily municipal solid waste but also most industrial solid wastes.
Municipal solid waste is an exceedingly difficult fuel to burn efficiently and cleanly because its different elements have such different combustion characteristics. For example, the wet leaves, having a high moisture content, must be thoroughly dried before combustion can commence, the magnesium chips are highly combustible and will burn explosively at an extremely high temperature of about 5500.degree. F., and the inerts such as glass will melt and corrosively attack the walls of the incinerator.
Fluidized bed combustors are well known in the art and are ideally suited to burning municipal waste. A bed of dry sand, perhaps three feet deep, is levitated by air blowing up through it, introduced by a large number of small orifices at its base. This airflow causes the sand to be partially suspended, giving it the property of partial weightlessness. The result is that the sand moves about freely, similar to a liquid. In operation, the sand is preheated above the auto-ignition temperature of average municipal solid waste and the waste is added to the sand bed where it burns within the bed supplying heat to the sand to make up for the cooling effect of the fluidizing air. The bubbling fluid bed is a large churning mass of red hot fine particulate, with air blowing up through it, which surrounds each component of the solid waste mixture, individually providing uniform heating and airflow for as long as required to combust it. Thus, a rubber heel can take perhaps five minutes to burn to completion while a dry piece of paper will burn in a second. Consequently, the fluid bed combustor has the property of variable retention times for fuels of varying combustion characteristics assuring complete burnout of the waste fuel.
Furthermore, combustion rates of solids in a fluid bed are considerably enhanced by the turbulent hot sand rubbing against the fuel surfaces. Heat transfer from the particulate to the burning surface can be five times greater than in a conventional burner and the particulate grinds char from the burning surface, exposing virgin material for new combustion.
In addition, the fluid bed combustor can operate at a near uniform temperature and can get good combustion efficiency at temperatures of 1450.degree. F. which is below the melting temperature of glass and other inerts. Further, the large thermal mass of the sand acts to drive moisture from wet loads (the wet leaves or wet telephone books) and absorbs bursts of energy from active chemicals (the magnesium chips). It acts as a thermal flywheel, i.e. the bed remains at nearly uniform temperature (.+-.5.degree. F.).
Finally, the fluid bed combustor will suppress the formation and escape of gas pollutants from the burning solid waste by absorbing the sulfur and chlorine in the inert particulate of the bed. Alkaline oxides in the ash, which may come from added limestone or from kaolin in paper burned in the bed, will produce sulfate and chloride salts which remain in the bed.
All of these advantages of incinerating municipal solid waste in a fluid bed have not been realized in practice because of the difficulties of feeding the municipal solid waste into the fluid bed, distributing it uniformly throughout the bed and causing it to burn below the surface of the bed. Problems arise because most of the combustibles in municipal solid waste are plastics and other low density materials that burn relatively rapidly once ignited, and especially so in the interior of the fluid bed. The first problem is to get these light materials into the bed to initiate combustion. If the municipal solid waste is distributed over the surface of the bed, as is coal, the light material will sit on the surface of the bed and burn in the freeboard over the bed. The heat from combustion will not get into the bed, which will slowly cool until it drops below the ignition temperature of the paper, at which time the paper burning over the bed will no longer ignite.
In addition to disposal of solid waste, most municipalities have the added problem of disposing of sewage sludge, which has a very high moisture content and little hearing value. In general, it cannot be burned autogenously. If this sewage sludge were to be fed into a conventional incinerator it would put out the fire. However, because of the excellent combustion characteristics of the fluid bed combustor, it will be cleanly incinerated with perhaps some loss in steaming capacity due to its exothermic nature.
Brown et al in U.S. Pat. No. 4,196,676 teach a fluidized bed for burning log yard waste from a wood products plant whereby the bed burns large and small pieces of wood and is designed to purge itself of the large number of inerts (rocks, for example) that are fed to it along with the log yard waste. Fluidizing air is given a velocity vector toward one side of the combustor for moving inerts to that side. Removal of these inerts is essential because if they accumulated in the bed, the fluidizing action of the bed would cease.
Smith et al in U.S. Pat. No. 3,589,313 and Reese in U.S. Pat. No. 3,922,975 each teach burning municipal solid waste in a fluid bed combustor. However, in each reference the municipal solid waste is preprocessed, for the purpose of removing many of the inerts and making the waste more homogeneous. Then the waste is introduced into the fluid bed below its surface, entrained in air jet. Preprocessing of municipal solid waste is prohibitively expensive and delivering the waste into the fluid bed, below its surface, through the usual inlet pipes is not practical because of the large irregular size of many of its constituents.
It is, therefore, the primary object of my invention to provide a unique fluid bed combustor which will operate efficiently with raw, un-preprocessed, heterogeneous municipal waste and, in particular, light bulky combustible materials, burning the waste within the fluid bed of particulate.
It is a further object of my invention to provide the combustor with inert removal apparatus to continuously purge the bed of inert foreign objects.
Another object of my invention is to provide a combustor which is smaller and cleaner than current designs.